We are doing a Boil Test Per ASTM-F 1089 on passivated Medical Instruments made of SS 17-4 and SS 440B. I would like to know whether this Boil Test will deteriorate the Passive layer? I understand that all these Surgical Instruments are subjected to several Autoclave cycles in their life time. So I am seeking advice from the forum members on this.
Thank You.
Manoharan
Passivation is the use of a light coat of a protective material, such as metal oxide, to create a shell against corrosion.
Passivation processes are generally controlled by industry standards, the most prevalent among them today being ASTM A 967 and AMS 2700. These industry standards will generally list several typical "types" of passivation processes that can be used.
Passivation is achieved either by the use of a Nitric Acid-based passivating bath , or a Citric Acid based bath.
In surgical Instruments this metal oxide is usually chromium oxide layer. It is achieved by treating the item in 3 stages (or three tanks with different chemicals) 1st tank contains Citric and Sodium Bi-Sulphate, usual treatment time is 30 minutes for a lot of 100 to 500 items 2nd treatment is in second tank which contains Chromic Acid which has high chromium contents and the treatment time is usually 45 minutes and in the end in stage 3 item is treated in Alkline Sodium Hydroxide (to neutralize the item or parts).
The passivation process results in a Chromium Oxide protective shield. Cr2O3 adopts the corundum structure, consisting of a hexagonal close packed array of oxide anions with 2/3 of the octahedral holes occupied by chromium. Chromium(III) oxide is amphoteric. It is insoluble in water. So when item went through boil test where de-ionised water is used , it does not reacts with chromium oxide but if any part which is not passivized and has iron oxide on the outer layer instead of chromium oxide, gets reacted and results in visible stains on the surface.
This passivation layer gets stronger with the time, Cleaning and Autoclaving does not destroy this layer either, unless there is any contamination in cleaning solutions.
